My research group studies dynamic phenomena within complex
fluids using experimental, computational, and theoretical
tools. Complex fluids, encompassing suspensions of particulates,
emulsions, and polymeric solutions and melts, serve important
roles in biotechnology, nanotechnology, materials science,
and emerging industrial technologies. Efficient control and
processing of these materials requires an understanding of
their transport properties, yet complex fluids often demonstrate
unexpected and intriguing behavior under flow. Some specific
examples from our studies are described.

Macromolecules flowing within a confined geometry experience
hydrodynamic and thermodynamic forces that can cause conformational
changes and net migration of the molecules. Recent simulations
and theoretical calculations demonstrated that the direction
and extent of migration depends upon multiple factors, which
can have important implications for the transport and control
of polymers in microfluidic devices. We are exploring possibilities
for using the phenomena for separations and other applications.

Dynamics and rheology of nanorods and rigid
polymers

Keywords: Complex Fluids, Transport phenomena,
Nanosciences

Rigid polymers are widely used as high performance plastics
and examples of Brownian fibers can be found in the form of
macromolecules of biological origin and in nanotechnology
in the form of nanotubes and nanorods. Current work focuses
on eliminating the disparity between quantitative predictions
and measurements of the dynamic and rheological properties
of suspensions of Brownian rods and rigid polymers. This includes
resolving the scaling for the the long-time rotational diffusion
in semi-dilute suspensions and elucidating the mechanism for
shear thinning in suspensions in the limit of strong shear
and weak diffusion.

Structure and rheology in oscillating suspensions

Keywords: Complex Fluids, Transport phenomena

Suspensions of non-colloidal spheres in non-uniform flows
can demix due to hydrodynamic interactions. For oscillatory
flows of suspensions within a tube, experiments indicate that
the particles migrate toward the wall under some conditions
and that there is sometimes a segregation of particles along
the pipe axis. We have explored the origin of this unexpected
behavior using rheological and simulations studies. Previously
unidentified phenomena resulting from the experiments and
modeling efforts have included the irreversibility of the
suspensions (as reflected by long-time changes in the rheology)
even at small amplitudes of oscillation and non-monotonic
viscosities which arise from underlying phase changes in the
microstructure.

Instabilities in sedimentation of non-spherical
particles

Keywords: Complex Fluids, Transport phenomena

Mutual
hydrodynamic interactions among rigid rods sedimenting
in a viscous fluid create inhomogeneities in concentration
which, in turn, can enhance the mean sedimentation
rate beyond that measured in a dilute suspension.
This contrasts with sedimenting spheres, where increasing
concentration hinders the sedimentation rate. Continuing
work focuses on measuring the instability, refining
the models, and extending the concept to other particle
types. Improved models will aid design of separation
processes and understanding of natural sedimentation
processes.